West Nile encephalitis and West Nile meningitis are forms of severe disease that affect a person’s nervous system. Encephalitis refers to an inflammation of the brain, meningitis is an inflammation of the membrane around the brain and the spinal cord.
Click here for further explanation of WN meningitis and/or encephalitis.
Nile fever refers to typically less severe cases
that show no evidence of neuroinvasion.
WN fever is considered a notifiable disease, however the number of cases reported
(as with all diseases) may be limited by whether persons affected seek care, whether
laboratory diagnosis is ordered and the extent to which cases are reported to health
authorities by the diagnosing physician.
Clinical includes persons with clinical manifestations
other than WN fever, WN encephalitis or WN meningitis,
such as acute flaccid paralysis. Clinical/Unspecified cases
are those for which sufficient clinical information
was not provided.
the case definition (2004) for Neuroinvasive
and Non-Neuroinvasive Domestic Arboviral Diseases. From the CDC Epidemiology
Total Human Cases Reported to CDC: These numbers reflect both mild and severe human disease cases occurring between January 1, 2006 to December 31, 2006 as reported through May 22, 2007 that have been reported to ArboNET by state and local health departments. ArboNET is the national, electronic surveillance system established by CDC to assist states in tracking West Nile virus and other mosquito-borne viruses. Information regarding 2006 virus/disease activity is posted when such cases are reported to CDC.
4269 cases, 1459 (34%) were reported as West Nile meningitis or
encephalitis (neuroinvasive disease), 2616 (61%) were reported as
West Nile fever (milder disease), and 194 (5%) were clinically
unspecified at this time. Please refer to state
health department web sites for further details regarding state
high proportion of neuroinvasive disease cases among
reported cases of West Nile virus disease reflects
surveillance reporting bias. Serious cases are more
likely to be reported than mild cases. Also, the surveillance
system is not designed to detect asymptomatic infections. Data
from population-based surveys indicate that among all
people who become infected with West Nile virus (including
people with asymptomatic infections) less than 1% will
develop severe neuroinvasive disease. See: Mostashari
F, Bunning ML, Kitsutani PT, et al. Epidemic West Nile
Encephalitis, New York, 1999: Results of a household-based
seroepidemiological survey. Lancet 2001;358:261-264.